Permeable Shoulders With Stone Reservoirs

Transcription

1 Permeable Shoulders With Stone Reservoirs Requested by: American Association of State Highway and Transportation Officials (AASHTO) Standing Committee on the Environment Prepared by: David K. Hein, P.Eng. Principal Investigator Applied Research Associates, Inc. 100 Trade Center Boulevard, Suite 200 Champaign, Illinois and Eric Strecker, P.E., Aaron Poresky, P.E. and Dr. Robert Roseen, P.E. Geosyntec Consultants Marie Venner Venner Consulting (October 4, 2013) The information contained in this report was prepared as part of NCHRP Project 25-25, Task 82, National Cooperative Highway Research Program, Transportation Research Board. SPECIAL NOTE: This report IS NOT an official publication of the National Cooperative Highway Research Program, Transportation Research Board, National Research Council, or The National Academies.

2 Page ii Acknowledgements This study was requested by the American Association of State Highway and Transportation Officials (AASHTO), and conducted as part of the National Cooperative Highway Research Program (NCHRP) Project The NCHRP is supported by annual voluntary contributions from the state Departments of Transportation. Project is intended to fund quick response studies on behalf of the AASHTO Standing Committee on the Environment. The report was prepared by David K. Hein, P.Eng. and Lori Schaus, P.Eng. of Applied Research Associates, Inc., Eric Strecker, P.E., Aaron Poresky, P.E. and Dr. Robert Roseen, P.E. of Geosyntec Consultants and Marie Venner of Venner Consulting. The work was guided by a task group co-chaired by Frannie Brindle, Oregon DOT and Scott McGowen, California DOT. The project was managed by Nanda Srinivasan, NCHRP Senior Program Officer. The panel for this project included: Ms. Frannie Brindle (Oregon DOT), Mr. Scott McGowen (CA DOT), Mr. Andrew Gisi (KSDOT), Mr. Kent Hansen (National Asphalt Pavement Association), Mr. Gary Lantrip (Texas DOT), Mr. Robert Rodden (American Concrete Pavement Association), Mr. Brian Toombs (Burgess and Niple. Incorporated), Mr. Njoroge W. Wainaina (NC DOT). The FHWA liaison was Brian Beucler. Disclaimer The opinions and conclusions expressed or implied are those of the research agency that performed the research and are not necessarily those of the Transportation Research Board or its sponsors. The information contained in this document was taken directly from the submission of the author(s). This document is not a report of the Transportation Research Board or of the National Research Council.

3 Page iii Abstract The objective of this study is to evaluate the suitability of using permeable pavement for roadway shoulder applications. Permeable pavement systems are designed to minimize environmental impacts, stormwater runoff, and flooding and to treat or remove pollutants by allowing stormwater to infiltrate through the pavement in a manner similar to pre-development hydrologic conditions State Departments of Transportation (DOTs) and other municipal organizations have expressed interest in permeable shoulders to assist in the overall management of stormwater. Water from the surface of the roadway would flow into the permeable shoulder into a stone reservoir to temporarily store and treat runoff before infiltration into the roadway subgrade soils and/or discharge to other stormwater conveyance and treatment systems. The primary benefits of permeable pavements are to reduce stormwater runoff volume, reduce stormwater runoff peak flows, reduce surface ponding, reduce stormwater pollutant load, decrease downstream erosion and increase groundwater recharge. Careful consideration of design features and construction techniques are necessary to ensure their success. Key design features include a careful assessment of the permeable pavement site and its surrounding land use to ensure that the pavement surface does not become contaminated with sand/dust or vegetative matter. A rational assessment of the traffic to which the pavement will be exposed will permit the designer to ensure that the pavement has sufficient structural capacity for its design life. A hydrological design taking into account rain water landing on the pavement and water shed from the highway lanes can be accommodated into the permeable pavement and then properly treated for water quality improvements and permitted to exit the pavement either through infiltration into the subgrade or controlled through underdrains. Construction processes and techniques should consider the protection of the permeable pavement from contaminants during construction and to ensure that the pavement is able to accommodate both vehicle loading and water infiltration and exfiltration in accordance with the pavement design. Finally, with all pavements, maintenance practices should including occasional vacuum sweeping to ensure the longevity of the permeable surface with repairs completed to address any localized deficiencies.

4 Page iv Table of Contents Table of Contents... iv List of Tables... v List of Figures... v List of Appendices... vi 1. Introduction Scope Structural and Hydrologic Design Construction Issues Maintenance Standards Permeable Pavement Technology Permeable Pavement System Porous Asphalt Pervious Concrete Permeable Interlocking Concrete Pavement Permeable Shoulder Feasibility Decision Criteria Permeable Shoulder Feasibility Decision Matrix Structural and Hydrological Design of Permeable Pavements Design for Pavement Structural Capacity Porous Asphalt and PICP Structural Design Pervious Concrete Structural Design Hydrologic Design of Permeable Shoulders General Hydrologic Analysis Framework Design Hydrologic Calculation Framework for Simple Volumetric Sizing Approaches (Presumptive Sizing) Design Hydrologic Calculation Framework for Design Event Hydrograph and Continuous Simulation Sizing Approaches (Performance Based Sizing) Design for Cold Climate Frost Considerations Balancing Structural and Hydrological Designs Permeable Shoulder Use and Configurations Conceptual Designs Detailed Design Element Guidance and Discussion Shoulder Construction New Construction Sequencing Retrofit Construction Sequencing Installation and Materials Subgrade Preparation Geosynthetic Applications and Installation Drainage Features Aggregate Stone Reservoir Compaction Permeable Pavement Surface Installation Porous Asphalt Pervious Concrete Permeable Interlocking Concrete Pavement... 53

5 Page v 9.6 Edge Restraints, Loss of Support and Interface with Roadway Pavement Expansive Soils and Fill Conditions Selection of Materials with Consideration of Potential Groundwater and Surface Water Quality Impacts Contractor Certifications and Experience Quality Control and Assurance Permeable Pavement Cost Permeable Pavement Maintenance Guidelines Preventive Maintenance Winter Maintenance Considerations Permeability Restoration Permeable Shoulder Structural Rehabilitation Permeable Shoulder Maintenance for Water Quality and Hydrologic Performance Future Research References List of Tables Table 4.1. Decision matrix considerations Table 4.2. Permeable shoulder feasibility decision matrix (with example project scores) Table 5.1. Analysis and design approaches for different hydrologic design goals Table 5.2. Example methods/tools for hydrograph generation Table 7.1. AASHTO geotextile requirements for separation Table 7.2. AASHTO subsurface drainage geotextile requirements Table 7.3. AASHTO overlap requirements for geotextiles Table 9.1. Recommendations for supplemental drainage elements Table 9.2. Potential elevated risks and associated material recommendations Table Examples of deficiencies and maintenance and restoration practices List of Figures Figure 1.1. Ponding on highway shoulder Figure 3.1. Turf pavers providing structural support and vegetative growth/permeability Figure 3.2. Permeable pavement infiltration types Figure 3.3. Porous asphalt pavement... 7 Figure 3.4. Pervious concrete pavement Figure 3.5. Permeable interlocking concrete pavement Figure 5.1. Structural and hydrological design flowchart Figure 5.2. Hydrological design process Figure 6.1. Basic permeable shoulder configuration Figure 6.2. Conceptual strengthened permeable shoulder Figure 6.3. Conceptual channeled permeable shoulder Figure 9.1. Installation of a geomembrane Figure 9.2. In-place geomembrane (geotextile ready to be folded to enclose the shoulder) Figure 9.3. Typical underdrain outlet detail (rural section) Figure 9.4. Typical underdrain outlet detail (urban section) Figure 9.5. Observation well detail

6 Page vi Figure 9.6. Example of a curb cut-out Figure 9.7. Appropriate compaction equipment for open graded aggregate Figure 9.8. Inappropriate (too small for size of project) for compaction Figure 9.9. National Asphalt Pavement Association Guidelines Figure Portland Cement Association Guidelines Figure Interlocking Concrete Pavement Institute Guidelines Figure Completed porous asphalt installation Figure Pervious concrete placement, jointing and curing Figure Typical permeable interlocking concrete pavement installation Figure Pavement failure due to water erosion List of Appendices Appendix A - Literature Review Appendix B - Lessons Learned

7 Page 1 1. Introduction The objective of this study is to evaluate the suitability of using permeable pavement for roadway shoulder applications. This research is also in direct support of the Moving Ahead for Progress in the 21 st Century (MAP-21) Act, in that Section 503 promotes the adoption of permeable, pervious or porous paving materials, practices and systems that are designed to minimize environmental impacts, stormwater runoff, and flooding and to treat or remove pollutants by allowing stormwater to infiltrate through the pavement in a manner similar to pre-development hydrologic conditions [MAP ]. State Departments of Transportation (DOTs) and other municipal organizations have expressed interest in permeable shoulders to assist in the overall management of stormwater [NCHRP interviews, NCHRP 25-25/82 research statement]. Water from the surface of the roadway would flow into the permeable shoulder into a stone reservoir to temporarily store and treat runoff before infiltration into the roadway subgrade soils and/or discharge to other stormwater conveyance and treatment systems. The primary benefits of permeable pavements are to reduce stormwater runoff volume, reduce stormwater runoff peak flows, reduce surface ponding, reduce stormwater pollutant load, decrease downstream erosion and increase groundwater recharge. This would potentially alleviate the conditions shown in Figure 1.1. Figure 1.1. Ponding on highway shoulder. Potential challenges for success of permeable shoulders include: reduced structural capacity compared to conventional pavements, possible moisture weakening of adjacent roadway lanes (and shoulders themselves), potential maintenance issues, varying maintenance needs from permeable to impermeable surfaces, durability, perceived safety concerns and overall life-cycle costs, including initial construction and maintenance costs. 2. Scope This report provides guidance for permeable shoulders with stone reservoirs using various permeable pavement technologies with respect to structural and hydrologic design, construction issues and maintenance standards. Generically, pavements that are specifically designed to infiltrate water are

8 Page 2 referred to as permeable pavements; however, the common naming convention for permeable pavements differs by industry as follows: Asphalt industry Porous Asphalt; Concrete industry Pervious Concrete; and Interlocking concrete pavement industry Permeable Interlocking Concrete Pavement (PICP). The report assesses the following: Ability of porous shoulders to reduce downstream infrastructure requirements (either conveyance and/or water quality treatment; Various subgrade conditions likely to be encountered; Recommended construction materials; Changes to the surface drainage system; Safety issues; Maintenance needs; Potential adverse impacts to the mainline structural section and Potential other impacts of water infiltration into the shoulders. A checklist of considerations for where permeable pavement approaches are appropriate and not based upon the factors identified during the literature review is also provided. The literature review conducted as part of this research has been provided in Appendix A. 2.1 Structural and Hydrologic Design Design guidance accommodates various types of structural sections in the adjacent mainline, superelevation and grade, various mainline pavement and shoulder widths (tributary area), recommended applications, and conditions that would and would not support use of the permeable shoulder section. Additional parameters considered include the following: Compaction and the use of underdrain systems for poorly drained soils; Use of impermeable liners to line the stone reservoir; and Type of embankment or natural soils. The structural and hydrologic design analysis is used to provide guidance to designers and practitioners on the feasibility of utilizing permeable shoulders for roadway pavements. Guidance is also provided with respect to the hydrological considerations necessary to manage stormwater runoff effectively. 2.2 Construction Issues An important part of the scope of this report is an evaluation of construction techniques and issues related to permeable shoulders. These include elements associated with both new and retrofit/rehabilitation shoulder construction, construction materials, quality assurance guidance, and items related to underground utilities. The report provides general guidance for specifications and construction of permeable shoulders.

9 Page Maintenance Standards Proper and timely maintenance is considered extremely critical for permeable pavement systems. The surface should be properly monitored and maintained to provide a durable and safe driving surface. Maintenance practices can greatly affect the ability of the permeable pavement system to effectively infiltrate water. Additionally, winter maintenance for permeable and impermeable pavements has important differences that need to be understood [Roseen 2013]. This report will provide guidelines and recommendations for permeable pavement maintenance and restoration. 3. Permeable Pavement Technology National, state/provincial and municipal legislation regulating stormwater runoff in the United States and Canada has provided increased incentives for using permeable pavements. In addition, regulatory frameworks for implementation of sustainable design have embraced permeable pavement solutions. These regulations are often called low impact development (LID) or sustainable urban drainage systems (SUDS). Permeable pavements in some form have been around for centuries and in the literature since the early 1970s [Ferguson 2005]. Early turf systems were developed to allow passage of personal vehicles and occasional heavy vehicles to drive and/or park on non-roadways surfaces without permanently disturbing the surface of the ground. In order to prevent damage to the ground, it is reinforced using geogrids, cellular structures or block pavers such as those shown in Figure 3.1. These systems permit natural vegetative growth to continue while providing support for vehicular traffic. Figure 3.1. Turf pavers providing structural support and vegetative growth/permeability. More recently, permeable pavement systems have been designed to accommodate more frequent and heavier loading than those of turf systems. These have included porous asphalt, pervious concrete, and permeable interlocking concrete paver surfaced pavements. Their application has expanded to include walkways, trails, driveways, large commercial parking areas, alleys and roadways.

10 Page 4 This research focuses specifically on the application of permeable pavements for highway shoulder applications. Additionally, while turf and cellular confinement systems may be considered for some shoulder applications, the focus of this research is on conventional hard-surfaced permeable paving surfaces: asphalt, concrete, and pavers. The following sections provide an overview of permeable pavement systems. 3.1 Permeable Pavement System Permeable pavement systems consist of a surface with joints and/or openings that will freely allow water to infiltrate to the subgrade. The openings allow water from storm events to flow freely through the surface into a stone reservoir where it is collected and stored before it leaves the pavement structure. For low-infiltration rate soils, perforated drain pipes are often placed in the stone reservoir or subgrade to drain excess water, thereby functioning as a detention facility that provides treatment for removal of stormwater pollutants and allows some infiltration. Infiltration reservoirs in the subsurface are often used to maximize storage and infiltration and can be used to replace end of pipe volume control practices. For sites that do not allow for any infiltration, permeable pavement is designed with an impermeable liner that prevents water from entering the soil subgrade; water is detained, treated, and exits via underdrains. Such sites could be locations where concerns exist for groundwater contamination due high risk pollutant load, shallow depth to groundwater or bedrock, and contaminated soils. Permeable pavement systems can support vehicular or pedestrian traffic while minimizing stormwater runoff and recharging groundwater supplies. Research on permeable asphalt friction courses over impervious asphalt has demonstrated that permeable pavements are an effective method for reducing stormwater runoff and pollutants from urbanized areas and can function well with minimal maintenance [Eck 2012]. For fully permeable pavement systems, Initial surface infiltration rates in a parking lot installation exceeded 200 cm/hr (80 in/hr) [USEPA 2010], which provides effective passage for rainfall and adjacent runoff into the stone reservoir. Design pollutant removal efficiencies are on the order of 85 percent for total suspended solids (TSS), 35 percent for Total Phosphorus and 30 percent for Total Nitrogen [NCDENR 2012]. Like all permeable pavements, the surface will accept sediment thereby potentially decreasing its infiltration rate with time. The rate of decrease depends on sources of deposited sediment typically from ordinary use and unexpected soil erosion from adjacent surfaces or spilled or applied (i.e. road sanding) materials. Such reductions from normal use still render a surface that can infiltrate most rain events, as Eck et. al indicate. Gradual clogging of the surface layer can have the benefit of capturing some suspended solids that would otherwise be deposited into the stone reservoir and/or discharge from the underdrains. With regular maintenance, the solids that are captured near the surface can be more readily removed than sediment that accumulates in the stone reservoir. General configurations of permeable pavements based on subgrade infiltration are shown in Figure 3.2. Each of these can be further detailed to achieve the specific goals for an individual installation, including items such as: surface type, curbing and other support features, use of geotextile for layer separation or water filtration, outlet pipe location, downstream water volume and quality treatment, etc.

11 Page 5 a. Full Infiltration b. Partial Infiltration c. Low Infiltration Figure 3.2. Permeable pavement infiltration types. Figure 3.2a shows a full infiltration design. All stormwater is infiltrated into the subgrade below the pavement. Notably, this system does not require any additional stormwater features such as catchbasins, outlet pipes, stormwater management ponds, etc. This system is typically only used in areas where the subgrade materials have high infiltration rates. Figure 3.2b, shows a partial infiltration design. Water is encouraged to infiltrate the subgrade; however, excess water from higher intensity storms is removed from the pavement structure using outlet pipes to ensure that that the pavement system does not overflow. The outlet pipe under the permeable pavement is typically placed at the bottom of the stone reservoir layer or in a shallow trench near the top of the subgrade layer to prevent it from being damaged during construction. In a partial infiltration design, the surface discharge elevation can be set by the designer based on how much infiltration is allowable and desired for site conditions. The controlling discharge elevation can also be controlled by an upturned elbow design, weir box, or other outlet structure at the downstream end of the outlet pipe. These designs can be effective even in low conductivity soils and they are especially useful in replacement of end of pipe peak flow control practices such as retention ponds. Water is encouraged to infiltrate the subgrade; however, excess water from higher intensity storms is removed from the pavement structure using outlet pipes to ensure that that the pavement system does not overflow. The outlet pipe under the permeable pavement is typically placed at least 10 cm (4 in.) from the bottom of the stone reservoir layer to maximize infiltration. The depth of the reservoir and location of the underdrain is determined by storage and infiltration needs for the project. Figure 3.2c, shows a low infiltration design. This type of system may be used for low permeability subgrade and where infiltration is not desirable (i.e. contaminated or moisture sensitive soils). They are also used for applications where infiltration would be undesirable, such as: Water harvesting applications where water is stored for subsequent use; Brownfield sites where it is not desirable to have water flowing through contaminated subgrades; Areas where the subgrade is susceptible to frost heaving or swelling due to moisture content variations; Areas with underground infrastructure that could be damaged via increased infiltration; or Where subgrade exposure to saturated conditions would result in a reduction in subgrade strength requiring a very thick pavement structure to accommodate traffic loading or create slope stability issues.

12 Page 6 While water will move laterally in the stone reservoir layer in the absence of underdrains, a perforated underdrain pipe is commonly used in design. The purpose of the underdrain pipe and outlet are to (1) provide a preferred pathway for water flow that has lesser potential to re-suspend settled particles at the bottom of the stone reservoir, and/or (2) provide a means to control the rate and controlling elevation of discharge to provide enhanced infiltration and/or flow control and limit the maximum water depth to protect the surface of the pavement. The three primary permeable pavement surface types as designated by their industry associations include porous asphalt, pervious concrete and permeable interlocking concrete pavement. These surface layers are typically underlain by open graded aggregate layer(s) (stone reservoir). These aggregate layers provide structural capacity to accommodate the traffic loading and act as a reservoir to store and release stormwater as required by the designer. As a design variation to increase load carrying capacity, the pavement industry in the United Kingdom has published a guide to the design and construction of permeable pavements that includes a layer of asphalt concrete beneath the permeable surface [Interpave 2012]. To maintain permeability, holes are drilled in the asphalt concrete and filled with open graded aggregate. The asphalt concrete layer provides additional strength and stability to the permeable surface while the aggregate filled holes provide positive drainage to the underlying stone reservoir. 3.2 Porous Asphalt Conventional asphalt concrete is typically designed to accommodate vehicle wheel loading and transmit this load to the underlying pavement layers. It is designed to be as dense as possible to provide maximum strength, prevent water from infiltrating beneath the pavement, and resist deformation under traffic loading. These characteristics result in a pavement surface that is relatively impermeable. In order to increase its permeability for use in permeable pavement systems, the aggregate gradation is modified to increase the air void space resulting in higher permeability. Historically, open graded asphalt has been used as a surface for conventional dense graded pavements to reduce noise and vehicle water spray during rainfall events. This open graded friction course (OGFC) is placed in a thickness of about 25 mm (1 in.). Water entering the OGFC is not transmitted into the overall pavement structure but rather laterally to the edge of the roadway. Another historic application of porous asphalt is for the construction of an open grade drainage layer which may be placed under the dense graded asphalt or concrete layers to provide lateral drainage for water penetrating the surface. This water is transmitted laterally to subdrains typically placed longitudinally along the edge of the pavement or shoulder. One of the oldest porous pavement installations was constructed in 1977 at the Walden Pond State Reservation in Massachusetts. It currently is in good condition and exhibits little cracking or other distresses, although its permeability has been impacted by the lack of maintenance that most BMPs require [NAPA 2013]. Porous asphalt is now being used as a complete surface for the pavement structure for permeable pavement applications. The porous asphalt provides a stable wearing surface for the pavement while transmitting all of the stormwater through the surface to the underlying stone reservoir. The void space of porous asphalt is at least 16 percent, as opposed to 6 to 8 percent for conventional asphalt. Treated runoff is temporarily stored in the reservoir bed, a highly permeable layer of open graded clean-washed aggregate with about 40 percent void space. Unmodified porous asphalt has lower strength than conventional dense graded asphalt. For example, thin layers of OGFC used for highway

13 Page 7 surfacing are typically assigned no structural contribution to the pavement. Porous asphalt used in moderate to high traffic applications includes the use of admixtures to increase the strength and durability of the pavement. Porous asphalt used for permeable pavements is assigned a structural contribution between that of a dense graded base and dense graded asphalt concrete. Mix design procedures for porous asphalt are similar to those used for OGFC mixes. This may include admixtures such as anti-stripping agents to enhance asphalt cement/aggregate adhesion. Mix designs for conventional asphalt concrete typically involve the use of stiffer grade asphalt binders achieved with a styrene butadiene rubber (SBR) or a styrene butadiene styrene (SBS), and/or the addition of fibers, and a dense material to provide strength and rut resistance with sufficient asphalt cement coating to resist thermal cracking [NAPA 2008]. Porous asphalt mix deigns, focus on the provision of an open structure to provide infiltration capacity while being stable and resistant to rutting/deformation under traffic. To improve durability of porous asphalt, a drain down test is completed (which is not typical for dense graded mixes other than stone mastic asphalt) and often there is a minimum asphalt content to assure a thick film. A Cantabro Abrasion test is used to evaluate the mix design durability (ASTM D ). An example porous asphalt pavement is shown in Figure 3.3. Figure 3.3. Porous asphalt pavement. For more information regarding porous asphalt: Pervious Concrete Conventional concrete is designed to be as dense as possible to be strong and relatively impermeable. In order to provide permeability for use in a pavement system, the aggregate gradation is modified to increase the air void space. Similar to asphalt, pervious concrete has also been used as an open graded drainage layer beneath conventional highway surfaces. The strength of pervious concrete is less than that of conventional concrete and as such needs to be thicker than conventional concrete pavements for the same level of traffic. The structural design for pervious concrete is different than that of asphalt concrete and is discussed in Section 6. Mix design procedures for pervious concrete are similar to that of conventional concrete using compressive

14 Page 8 strength cylinders and flexural strength beams to assess structural capacity. An example pervious concrete pavement is shown in Figure 3.4. Figure 3.4. Pervious concrete pavement. For more information regarding pervious concrete: Permeable Interlocking Concrete Pavement There are several types of permeable interlocking concrete pavements (PICPs). Conventional interlocking concrete pavement (ICP) consists of high strength impermeable concrete blocks laid over a sand bedding course with sand-filled joints to assist in interlocking the blocks together. This type of system is not very permeable. In order to make it permeable, either the concrete block has to be permeable or the blocks must be constructed such that their configuration provides openings to provide permeability. Both the sand bedding and joint filler for PICPs is replaced with a stone chip material that provides both friction to prevent block movement and permeability to allow the infiltration of water. A key advantage of the PICP systems is that the blocks are hard and durable with compressive strengths in the order of 55 MPa (8,000 psi). An example PICP is shown in Figure 3.5. Figure 3.5. Permeable interlocking concrete pavement.

15 Page 9 For more information regarding permeable interlocking concrete pavement: 4. Permeable Shoulder Feasibility Decision Criteria The primary purpose of an outside, i.e. to the right of the travel lanes, highway shoulder is to provide a safety zone for emergency pull-off from the main highway lanes. The median shoulder (depending on the width), may also provide a safety zone or offset from median barrier systems. Shoulders may be used to carry mainline traffic during rehabilitation and maintenance operations and be used by other modes of transportation including buses, bicycles, etc. Shoulders also provide lateral support to the pavement structure and drainage of surface water away from the travelled portion of the roadway. Shoulder pavements may be constructed as granular surface, partially and/or fully paved hard surface depending on the highway classification and locations, e.g. urban versus rural. Permeable shoulders may provide all of the features above, but also provide the opportunity for stormwater management. While the use of permeable shoulders may have significant benefits in terms of stormwater management, their application is not be suitable for all situations. In order to be successful, consideration must be given to site specific conditions, design, construction and maintenance details. The following stepwise process is recommended for evaluating the suitability and opportunity for permeable pavement and identifying key factors that may influence design and effectiveness. Step 1 - Evaluate acceptability. The first step in determining the suitability of a project for permeable pavement shoulders is to determine if they are permitted by national and local regulations for the project location. The advantages of permeable shoulders are such that lack of prohibition should be taken as permissive or demonstrative (allowing a demonstration of the benefits). Step 2 - Evaluate opportunities and drivers. The next step is to evaluate whether the project provides the opportunity for any of the following general considerations: Desire or regulatory requirements to reduce the volume of stormwater runoff, reduce peak runoff flowrates, improve the quality of stormwater runoff, and/or and address other related run-off issues such as effluent temperature; Increase safety via reducing splash and/or surface ponding when vehicles are using the shoulder; Potential for reduction in future stormwater management costs by modifying pavement design for stormwater management and eliminating end of pipe structures; and Incentives (financial, environmental benefits, sustainability achievement, etc.). Step 3 - Evaluate benefits, risks, and technical design factors. If the analysis in Step 1 and 2 indicates that the use of a permeable shoulder may be suitable for a particular project, additional specific considerations should be analyzed to evaluate the anticipated benefits and risks associated with its use. To determine the suitability of a project for permeable pavement shoulders, the key factors specific to the project should be considered. Based on their importance in overall decision making, these factors can be divided into primary, secondary, and other considerations which may impact the

16 Page 10 decision to use permeable shoulders for a particular project. Primary considerations (i.e., fatal flaws or major design challenges) are those that would have an overriding influence on the decision to move forward with the project. Secondary considerations are those that have a lesser influence and usually are taken into account as part of the design process when there are no overriding considerations. These factors may diminish the performance or acceptability of permeable shoulders or may require additional design provisions (and associated costs) to avoid risks. Other considerations may have some influence on the decision to include permeable shoulders for a particular project. The primary considerations should generally be weighted the highest to reflect their importance in moving forward with the project, while secondary and other considerations are useful to prioritize between sites and influence design, but are not generally fatal flaws. 1. Primary Considerations Availability of capital funding Status of environmental approval Regulatory requirement to consider use of permeable shoulders Safety Significant longitudinal grades (>5 percent) Depth of water table Geotechnical risks Groundwater contamination risk 2. Secondary Considerations Stringent receiving water quality standards Sand use for winter maintenance Run-on from adjacent areas with exposed soils Low soil infiltration rates Target design volumes and runoff rates Complexity of geometric conditions (super elevation, shoulder width available, number of lanes, etc.) Risk of flooding Mandates for stormwater quality control Mandates for drainage and peak flow control Maintenance protocols Shoulder utilization 3. Other Considerations Interest in innovation Presence of utilities Impact of unknown site conditions Risk of accidental chemical spill Owner experience and resources This list of considerations was developed for illustrative purposes. This is not an exhaustive list, but rather reflects typical needs and expectations. Constraints and project-specific considerations should

17 Page 11 be added or deleted as necessary. The individual weighting of the considerations should be modified to reflect local agency needs and expectations. A description of each of the considerations is summarized in Table 4.1. Importance Level Primary Considerations Availability of capital funding Status of environmental approval Safety Significant longitudinal grades Depth of water table Geotechnical risks Table 4.1. Decision matrix considerations Description The initial capital construction cost of permeable pavement is typically higher than for conventional pavement. Overall long term life-cycle costs can be very competitive if consideration is given to stormwater quality and quantity benefits and the costs of constructing and maintaining other stormwater treatment facilities in the right-of-way. In some jurisdictions, permeable pavement may not be permitted or may require additional environmental approvals. On the other hand, desirability for stormwater quality and quantity management can drive regulatory acceptance of and even advocacy for permeable pavements. Ability to accommodate safety features such as rumble strips, vegetative growth, etc. Grades of more than 5 percent may pose significant design challenges. A stone reservoir with a sloped base may be much less effective at promoting infiltration than a flat reservoir because water will seek a level surface and pond. Significant longitudinal grades may require relatively costly design features such as regular cut-off walls or below grade shallow slopes with step-downs to provide needed level of infiltration to achieve design goals. Permeable pavements should not be used in areas where the water table is within 0.6 m (2 ft) of the top of the soil subgrade. It must be possible to drain water entering the subgrade. Note, this criteria is not intended to address potential for groundwater contamination as a result of high groundwater table or other soil factors (see groundwater contamination risk ) Depending on site-specific conditions, infiltration of water below a road shoulder may pose a range of geotechnical risks, such as reduced subgrade support, slope stability, scouring, etc. In Karst areas, increased below ground infiltration may cause sink holes. Geotechnical risks may introduce added design complexity and may necessitate the use of an underdrain and/or impermeable liner.

18 Page 12 Importance Level Groundwater contamination risk Secondary Considerations Stringent receiving water quality standards Sand use for winter maintenance Low soil infiltration rates Target design volumes and runoff rates Complexity of geometric conditions Description A variety of factors influence the potential for stormwater sources to contaminate groundwater, including soil characteristics, depth to groundwater, traffic volume, existing soil contamination, and application of salt for deicing. Where there is elevated potential for contamination, design features may be included or required to mitigate this risk, including soil amendment below shoulders, use of an underdrain, and/or a liner. While the presence of and need to protect nearby aquatic resources may provide incentives for the use of permeable pavements in cases, for some protected watersheds, cold water streams, and other receiving waters with stringent water quality standards, the level of treatment provided by permeable pavements (for water discharged from underdrains) may not provide adequate protection from stormwater quality impacts. In cases where infiltration is not feasible, permeable shoulders with underdrains may need to be coupled with an additional treatment system. In such a case, permeable shoulders may not provide sufficient benefit to justify their use. Winter sand may clog permeable pavement systems resulting in reduced system permeability. Soil infiltration rates influence performance of permeable shoulders for volume and peak flow reduction. Low soil infiltration rates may need to be supplemented with an underdrain to provide adequate drainage, which tends to reduce performance and increase costs compared to conditions where infiltration rates are adequate to support a design without underdrains. Due to geometric factors, permeable shoulders may be limited in terms of how much volume they can store and the maximum rate of sheet flow from travel lanes that can be captured in the permeable shoulder. Where target design volumes or intensities are large in comparison to available space, permeable shoulders may have a reduced effectiveness. A site-specific hydrologic analysis based on site rainfall patterns, roadway geometry (i.e., number of lanes vs. shoulder area), stormwater management goals (i.e., long term volume control vs. peak event mitigation) is recommended to evaluate whether permeable shoulders would be suitable for meeting design goals. Geometric constraints such as horizontal or vertical grades, presence of bridge structures, curbs, retaining walls, guiderails, etc.

19 Page 13 Importance Level Risk of flooding Mandates for stormwater quality control Mandates for drainage and peak flow control Maintenance protocols Shoulder utilization Other Considerations Interest in innovation Presence of utilities Impact of unknown site conditions Risk of accidental chemical spill Owner experience and resources Description Permeable shoulders may not be capable of conveying flows from peak storm events. Areas subject to frequent roadway flooding may require supplemental drainage features to ensure that the roadway surface is properly drained; however, it is likely that such drainage features would be required with or without the use of impermeable shoulders. Permeable pavements may contribute substantially to water quality improvement. Where regulations are in place requiring stormwater quality management, this may significantly incentivize the use of permeable shoulders. Permeable pavements provide stormwater management alternatives to more costly or complicated practices to provide drainage and peak flow control. In order to maintain their effectiveness in some areas, permeable pavement systems require mandatory maintenance practices such as vacuum sweeping, which may influence their applicability and desirability for a project. Some shoulders are used as driving lanes for specific conditions or circumstances, e.g. evacuation routes, rush hour traffic, pullovers for passing, high occupancy vehicle routes, emergency vehicles, etc. Design of shoulders for moderate to heavy traffic use would require additional considerations and additional costs. Utilizing nontraditional surfaces for stormwater management provides opportunities for innovation. The design and construction of permeable shoulders may be problematic or require additional design features, such as cutoff walls, in areas where utilities are present along the roadway shoulders. Variability of soil conditions, presence of organics, potential for frost heave, etc. may impact shoulder pavement performance. While spills are relatively uncommon and tend to occur at low volumes they can typically be retained in the permeable shoulder. Based on typical volume of chemical spill being less than the capacity of water quality facilities, it is likely that spills would be captured by the stone reservoir. This would be a benefit in some respects; however, if conditions are conducive for migration of spilled substances to groundwater, the use of permeable shoulders could increase the potential for major soil/groundwater cleanup efforts, depending on the downstream conditions. Higher weighting may be considered in relation to the groundwater quality risks posed by spills (i.e., soil type, depth to groundwater). The use of permeable pavements for roadway shoulders is very limited at present.

20 Page Permeable Shoulder Feasibility Decision Matrix To assist in evaluating the suitability of projects for the use of permeable shoulders, a project suitability matrix (template) was developed (Table 4.2) which could be tailored for individual user needs. The matrix includes the considerations outlined above with appropriate weighting factors for each group. Within each group, the individual consideration items also are given weighting factors. Each factor should be assessed using specific criteria of the owner s needs and expectations for the project. Once the factor is rated, the total scores are summed on a scale of 0 to 100. A suggested total score evaluation metric includes if the score totals less than 65; the project is not considered a good candidate for permeable shoulders. Between 65 and 75, the project can be considered for permeable shoulders. Scores over 75 indicate that the project is well suited for permeable shoulders. This scoring evaluation should be vetted by DOTs and adjusted as necessary for their own conditions. In the example shown in Table 4.2, the primary considerations have been given a category weighting of 60 points; the secondary considerations are weighted at 30, and other considerations are weighted at 10. When considering the primary factors, there was a preference for selecting projects where funding was available, where there are minimal environmental issues and where there is sufficient depth to the water table to provide adequate drainage. In terms of secondary factors, there is a clear mandate for stormwater quality and quantity improvements with minimal maintenance and operational concerns, favoring the use of permeable pavements. The other considerations category provides only a minimal contribution to the decision weighting. These weighting factors can be adjusted by DOTs to better reflect their goals and objectives.

21 Page 15 Table 4.2. Permeable shoulder feasibility decision matrix (with example project scores) 1. Primary Considerations Part 1 Weighting: 60 Project Scoring Guidelines Consideration Project Score Weighting Weighted Score A B C Favorable for Permeable Shoulders <<====================>> Not Favorable for Permeable Shoulders Availability of Capital Funding B Project funded; requirement to No specific funding available; no Need to justify funding implement requirement to implement Status of Environmental Approval B Approved Approval pending Application required Safety A Minimal safety issues Safety issues can be addressed Significant safety issues Significant Longitudinal Grades B Grades < 2 percent Grades of 2 to 5 percent Grades > 5 percent Depth of Water Table B Water table m (2-5 ft) below Water table > 1.5 m (5 ft) below subgrade subgrade Water table < 0.6 m (2 ft) below subgrade Geotechnical Risks B Low complexity Medium complexity High complexity Groundwater Contamination Risk A Low risk Elevated Risk High risk Total See Table 4.1 for guidance on scoring Weighted Total Score: Secondary Considerations Part 2 Weighting: 30 Project Scoring Guidelines Consideration Project Score Weighting Weighted Score A B C Favorable for Permeable Shoulders <<====================>> Not Favorable for Permeable Shoulders Stringent Water Quality Standards B Regulations in place Limited restriction No restrictions Sand use for Winter Maintenance B No sand use Used < 2 times/year Used > 2 times/year Low Soil Infiltration Rates A Infiltration < 12 mm/hr (1/2 in./hr) Infiltration >12mm/hr (1/2 in./hr) < 40 mm/hr (1.5 in./hr) Infiltration > 40 mm/hr (1.5 in./hr) Target Design Volumes and Runoff A Frequent/non-intense storm Moderate frequency/intensity Intense storms Complexity of Geometric Conditions A Minimal geometric restrictions Some geometric challenges Significant geometric restrictions Risk of Flooding A None Occasional Frequent Mandates for Water Quality B Water quality concerns Some water quality issues No concerns Mandates for Stormwater Management A Stormwater management concerns Some stormwater management issues No concerns Maintenance Protocols C Proactive maintenance Reactive maintenance Minimal maintenance Shoulder Utilization B Use for emergency stopping only Occasional use for traffic Regular use by traffic Total See Table 4.1 for guidance on scoring Weighted Total Score: Other Considerations Part 3 Weighting: 10 Project Scoring Guidelines Consideration Project Score Weighting Weighted Score A B C Favorable for Permeable Shoulders <<====================>> Not Favorable for Permeable Shoulders Interest in Innovation B Regular innovation implementation Innovation encouraged Minimal interest Presence of Utilities B None Non-critical utilities Critical utilities Impact of Unknown Site Conditions B Site conditions well known Some site information available No site specific information available Risk of Accidental Chemical Spill A Limited exposure Elevated risk of spills and elevated risk Elevated risk of spills or elevated risk of gr of groundwater contamination Owner Experience and Resources C Significant owner experience Limited owner experience No owner experience Total See Table 4.1 for guidance on scoring Weighted Total Score: 6.0 Sub Totals 1. Primary Considerations From Decision Range To Implement Alternative 2. Secondary Considerations No 3. Other Considerations Can Consider Grand Total Project Score Yes Decision Can Consider

22 Page Structural and Hydrological Design of Permeable Pavements The design of permeable pavements requires the consideration of both structural and hydrological components as shown in Figure 5.1. The structural design of the pavement is completed to determine the thickness of the various pavement components that are necessary to support the intended design traffic while protecting the subgrade from permanent deformation. The hydrological design determines the key design elements necessary to infiltrate rainwater and surface runoff into the pavement and hold and/or detain and filter the water to achieve the stormwater management objectives. An optimal pavement design is one that is just strong enough to accommodate the design traffic and has the minimum hydrological features to provide water quantity and quality management. For example, if a 230 mm (9 in.) stone reservoir is needed to provide adequate structural integrity and a 230 mm (9 in.) stone reservoir is needed to provide the storage needed to meet hydrologic goals, and then the design would be considered optimal. Frequently; however, designs are controlled by structural considerations and therefore may be over designed for hydrologic performance. Figure 5.1. Structural and hydrological design flowchart. 5.1 Design for Pavement Structural Capacity [Smith 2011] The most common structural analysis procedure for porous asphalt and permeable interlocking concrete pavement follows the requirements of the American Association of State Highway and Transportation Officials (AASHTO) Guide for the Design of Pavement Structures [AASHTO 1993]. Pervious concrete structural design is based on the StreetPave system as modified by the American Concrete Paving Association (ACPA) [ACPA 2012]. Brief descriptions of the design methods used with these systems is provided in the following sections.